The shift from laboratory to courtroom means the outcome will hinge not on scientific standards of evidence but on a legal standard of plausibility -- what one lawyer for the families called "50 percent and a feather." That may make it easier for the plaintiffs to sway the panel of three "special masters," which is why the decision could not only change the lives of thousands of American families but also have a profound effect on the decisions of parents around the world about whether to vaccinate their children.

"There is a difference between scientific proof and legal proof," [Kevin Conway] said. "One is 95 percent certainty, and the other is . . . 50 percent and a feather."

"We are absolutely confident Rachel's vaccines have nothing to do with her autism," he said. "If we could roll back the clock, we would give her all the vaccines again."